My U.S. Pat. No. 5,442,560, granted on Aug. 15, 1993, titled "Integrated Guidance System and Method for Providing Guidance to a Projectile on a Trajectory" and incorporated by reference, considers the need for precise data representing the rotation and velocity of a missile in a missile guidance control or system. It shows a so-called "strap down" inertial measurement unit that is used as an element in missile guidance control using a computer that requires IMU initialization prior to vehicle movement. The patent explains an advancement that uses a core strap down IMU 13 containing accelerometers A1,A2, A3 and gyroscopes G1, G2, G3 that are not rotated for initialization, like the prior art. Instead, the core IMU is aided by a fourth accelerometer A4 and star sensor 31 that are rotated by a motor 51 to initialize a computer or signal processor 39 for calibration of the fourth accelerometer in the gravity field prior to launch of the projectile or "vehicle". The star sensor is used to help locate the missile orientation in space. The fourth accelerometer may be a PIGA (Pendulous Integrating Gyroscope Accelerometer) which may be accurately calibrated on the earth's gravity before launch. The fourth accelerometer aid the strap down core IMU for accuracy of the total system.
Regardless of whether the core IMU is strapped down all the time, as explained in the patent, or after launch, during a reentry mode the vehicle (e.g., projectile or missile) is subjected to substantial forces that induce vehicle structural changes that in turn produce changes in the IMU output, degrading the accuracy of vehicle guidance. That is, while the core IMU is locked in place, it may move as the vehicle is stressed, producing errors that degrade total system accuracy.